1. Field of the Invention
This invention relates to a differential unit, particularly suitable for use in motor vehicles as described hereafter, which incorporates a controllable friction assembly for restricting the differential action of the unit.
More particularly, the invention relates to a differential unit comprising:
a housing; PA1 a drivable differential carrier supported in the housing for rotation about an axis; PA1 two output elements rotatable about said axis relative to the differential carrier and connected to respective output shafts; PA1 differential gears rotatably supported by the differential carrier and having driving connection with the output elements to permit differential rotation thereof; PA1 a friction assembly axially supported by one rotatable element of the differential, and axially loadable by an actuating means to cause a frictional restriction of said differential rotation of the output elements; PA1 the actuating means comprising an abutment ring axially fixed relative to the housing; an actuating ring movable axially relative thereto, one of the abutment and actuating rings being movable angularly relative to the other; and means for producing an axial movement of the actuating ring upon said relative angular movement between the rings, said means comprising cooperating surfaces provided on said abutment ring and actuating ring, and rolling members disposed therebetween.
Such a differential unit will hereafter be referred to as a differential unit of the kind specified.
The friction assembly preferably comprises a number of plates rotationally fast with said one rotatable element of the differential, interleaved with a number of plates rotationally fast with another element of the differential rotatable about said axis.
2. Description of the Prior Art
In JP 64-3148 there is disclosed a differential unit which is generally of the kind specified except that the friction assembly is arranged to be axially loaded by way of a hydraulic cylinder incorporated into the differential unit. Such an actuating means for the friction assembly is effective in terms of its function and behaviour, but relatively expensive, even if the differential unit is to be used in a motor vehicle which already has an hydraulic system for power steering or suspension purposes for example.
DE-OS 3707115 discloses a differential unit with a controllable frictional restriction of its differential action, wherein electro-magnetic means is used for loading the friction assembly. However, electro-magnetic actuation has the disadvantage that an acceptable force to load the friction assembly can only be produced within a relatively narrow wear range of the plates in the friction assembly, because otherwise the coil and armature upon which it acts are not in the correct relative position for a satisfactory force to be exerted.
Another differential unit wherein a friction assembly is actuated electro-magnetically is disclosed in DE-OS 3733771. In this differential, the friction assembly may not be able to exert sufficient force adequately to resist the differential and the unit is very bulky which renders it difficult to fit in a vehicle with a transversely mounted engine-transmission unit. Further, there can be fluctuations in the torque of the friction assembly.
A further design of differential unit is proposed in US 4,805,486, wherein the actuating mechanism for the friction assembly comprises coaxially disposed plates having circumferentially extending inclined grooves provided on opposed faces thereof, with rolling members (balls) disposed in the grooves. One of the plates is firmly connected to the housing of the differential unit, whilst the other is movable angularly by an electric motor through a toothed reduction gear arrangement.
The arrangement is such that when the electric motor is not energised, the plates remain apart from one another. This is undesirable in that if there is any failure the differential may remain in the condition wherein the differential action between its output elements is restricted. Further, the frictional forces which arise between the toothed expansion plate and the pinion engaged thereby cause disadvantageous hysteresis effects during adjustment of the force applied to the friction assembly.